Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders typically comprise small particles, e.g., between 80 mesh U.S. standard screen size (180 microns) and about 5 microns.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion thermal spray gun, the carrier gas, which entrains and transports the powders, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations. A modified type of plasma gun utilizes a transferred arc between the gun and the substrate.
The material alternatively may be fed into a heating zone in the form of a rod or wire. In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, such as a combustion flame, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated. In an arc wire gun two wires are melted in an electric arc struck between the wire ends, and the molten metal is atomized by compressed gas, usually air, and sprayed to a workpiece to be coated. The rod or wire may be conventionally formed as by drawing, or may be formed by sintering together a powder, or by bonding together the powder by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the powder to be sprayed in finely divided form.
Self-fluxing alloys of nickel and cobalt are quite common for hard facing coatings. They contain boron and silicon which act as fluxing agents during the coating operation and as hardening agents in the coating. A common method of processing such alloys is by thermal spraying.
Usually self-fluxing alloys are applied in two steps, namely thermal sprayed in the normal manner and then fused in situ with an oxyacetylene torch, induction coil, furnace or the like. The fluxing agents make the fusing step practical in open air. However, the alloys may also be thermal sprayed with a process such as plasma spraying without requiring the fusing step, but the coatings are not quite as dense or wear resistant. Generally self-fluxing alloy coatings are used for hard surfacing to provide wear resistance, particularly where a good surface finish is required since the fusing produces a coating having very low porosity.
A typical self-fluxing alloy composition of nickel or cobalt contains chromium, boron, silicon and carbon. An alloy may additionally contain molybdenum, tungsten, copper and/or iron. For example U.S. Pat. No. 2,875,043 discloses a spray weld alloy composed of (by weight) up to 20% chromium, 1 to 6% boron, up to 6% silicon, up to 10% iron, 3 to 10% molybdenum, 3 to 8% copper, up to 1.5% carbon, the remainder at least 40% nickel. Similarly, U.S. Pat. No. 2,936,229 discloses a cobalt alloy containing 1.5 to 4% boron, 0 to 4% silicon, 0 to 3% carbon, 0 to 20% tungsten and 0 to 8% molybdenum.
If very high wear resistance is needed a carbide such as tungsten carbide is added as described, for example, in British Pat. No. 867,455. These carbide-containing alloys are generally difficult to grind finish and are harder to fuse than the self-fluxing alloys without carbide.
As illustrated in the above-mentioned British patent, a tungsten carbide typically is combined with a cobalt or nickel binder in an amount, for example, of about 12 percent by weight. The cobalt or nickel may be combined with the carbide by sintering or alloying. Alternatively the metal may be clad onto the carbide as taught in U.S. Pat. Nos. 3,049,435 and 3,254,970. The first of these patents discloses nickel clad tungsten carbide blended with self-fluxing alloy powder.
The above-mentioned U.S. Pat. No. 3,254,970 discloses various composite flame (i.e., thermal) spray powders formed by cladding including nickel clad cobalt-tungsten carbide and nickel clad nickel-titanium carbide (Examples 10 and 14). The patent also discloses the cladding of various metals with copper (e.g., Examples 25 and 26). A process for copper cladding powders of a number of metals and oxides as well as tungsten carbide and titanium carbide is taught in U.S. Patent No. 4,309,457.
Chromium carbide (Cr.sub.3 C.sub.2) powder is known for use in the thermal spray process, as is chromium boride. One form, nickel clad chromium carbide, has been thermal sprayed, for example, in a blend with a self-fluxing alloy powder and a nickel-aluminum composite powder as a product sold by Metco Division of The Perkin-Elmer Corporation, Westbury, N.Y., as "Metco 430NS".
In view of the foregoing, a primary object of the present invention is to provide a novel form of chromium carbide and chromium boride thermal spray powder.
A further object of this invention is to provide an improved self-fluxing alloy-containing composition with a novel form of chromium carbide or chromium boride, for producing thermal spray coatings characterized by ease of fusing and grind finishing. Another object is to provide a thermal spray process for producing wear resistant coatings characterized by ease of fusing and grind finishing.